zippyshare (download & probe): fix HTTP 404 case
docs (plowprobe.1): fix english typo
zippyshare (download): fix (upstream modified)
plowprobe: add message if provided url can be used with plowlist
megashares (download & probe): fix (upstream modified)
plowdown: change --printf behavior, add --run-before/--run-after

1. --printf switch has now the same behavior than plowlist/plowprobe/plowup

It prints results (on stdout) for each successful download.
Default string is: "%F" (final filename with full path if output directory has been specified and with optionnal suffix if required by --no-overwrite).
For example:
- /media/foo
- /media/foo.1
- bar
- bar.1

Now final link is always processed, this is not instead.

Exported cookie files (with %c) are now correctly numbered:
plowdown-cookies-0001.txt
plowdown-cookies-0002.txt
...

2. New --skip-final flag option

Don't process final link (returned by module), just skip it (for each link).

3. The long awaited options: --run-before & --run-after

--run-before is invoked before call module download function. Depending return value download can be skipped.
Here is an example:

| #!/bin/bash
| # This script is invoked with:
| # $1: module name
| # $2: download URL
| # $3: cookie (empty) file given to download module function
| # stdout will be trashed.
|
| declare -r ERR_NOMODULE=2
|
| # You can print messages to stderr
| echo "===[Pre-processing script for $1]===" >&2
| echo "Temporary cookie file: $3" >&2
|
| # We want to skip all downloads from hotfile hoster
| if [ "$1" = 'hotfile' ]; then
|     exit $ERR_NOMODULE
| fi
|
| exit 0

--run-after is invoked after module download function with succesfull return status (0).
Here is an example:

| #!/bin/bash
| # This script is invoked with:
| # $1: module name
| # $2: download URL
| # $3: cookie file fulfilled by download module function
| # $4: final download url
| # $5: final filename
| # stdout will be trashed.
|
| # You can print messages to stderr
| echo "===[Post-processing script for $1]===" >&2
| echo "Final URL: $4" >&2
| echo "Filename: $5" >&2
|
| exit 0

Note: If --skip-final is specified, --run-after is called.

5. Extra notes:
- "--exec" command line option has been dropped, use "--skip-final" with "--run-after".
- Documentation update will come later.
- "--cookies" command line option has been kept, but prefer using "--run-before".